Knowledge Bank

In the one ring N-heterocyclics, e.g. pyridine and the three diazines, the lowest singlet electronic states are $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}W$ $(n\pi \ast$ type) and are clearly separated from the $(\pi \pi \ast )$ $states.1$ In the two ring N-heterocyclics, the be quite different. In the diazanaphthalenes (quinoxaline and phthalazine, the 1,4- and 2,3-diazanaphthalene respectively), the lowest singlet state is a clearly resolved (n, $\pi \ast$) $state.2$ However in the case of quinoline and isoquinoline (1- and 2-azanaphthalene respectively), the lowest singlet state appears to be of the ($\pi ,\pi \ast$) type. Solvent dependence of the emission spectra proves this to be not the case. The total emission spectra of quinoline, isoquinoline, and quinoxaline were compared with those of naphthalene, and 2-chloronaphthalene, all studied in both hydroxylic and nonhydroxylic rigid glass solutions at $77\circ K$. The fluorescence/phosphorescence yield ratios of naphthalene and chloronaphthalene were independent of solvent. For quinoline and isoquinoline, the ratio increased greatly in a hydroxylic solvent; actually, quinoline showed only a strong phosphorescence in a non-hydroxylic solvent, but a strong fluorescence and phosphorescence in hydroxylic solvent. For quinoxaline, only phosphorescence was observed in all solvents, as expected for a molecule with clearly separated lowest singlet state of $(n,\pi \ast )$ type. The above results can be interpreted as involving the interchange of the lowest $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}W(n,\pi \ast )$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Lb(\pi ,\pi \ast )$ states in the quinolines upon the formation of hydrogen bonded complexes. Such a possibility has been proposed for chlorophyll by several $authors.3,4$ The qualitative features are confirmed by the vapor spectrum studies of quinoline by Mataga $etal,5$ who found evidence for a lowest $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}W$ state. In all of the cases studied the phosphorescence is shown to be from a state. The mean lifetime of the phosphorescences were measured and found to be in the order naphthalene $>$ quinoline $$ isoquinoline $>$ quinoxaline, which can be understood by the expected differences of spin-orbital interactions in these molecules.